I. Field of the Invention
The invention is directed to an improved method of removing hydrogen sulfide from gaseous streams. More particularly, the invention is directed to a method of stabilizing a reaction solution containing at least one polyvalent metal chelated with a chelating agent to prevent the loss of the chelating agent and the metal from the reaction solution.
II. Description of the Prior Art
The removal of hydrogen sulfide (H.sub.2 S) from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in the pulping of wood, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide-containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, as well as constitute a significant health hazard to humans. Government regulations have increasingly imposed continuously lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all of the hydrogen sulfide under the penalty of an absolute ban on continuing operation of commercial plants which produce the hydrogen sulfide-containing gaseous streams.
The quantities of hydrogen sulfide in process gas streams are normally not very high. U.S. Pat. No. 3,071,433, dated Jan. 1, 1964 to Dunn, indicates that the stack gases obtained in the concentration of black liquor, the waste pulping liquor of the Kraft pulping process, contain from 500 to 2000 parts per million (ppm) of hydrogen sulfide. However, the odor of hydrogen sulfide can be detected by humans at a concentration of approximately 0.01 ppm. Consequently, an extremely efficient process for the removal of hydrogen sulfide is required to eliminate small amounts of noxious hydrogen sulfide from process gases.
It is known to effect the removal of hydrogen sulfide in an oxidation-reduction system by contacting the hydrogen sulfide-containing gas stream with a solution of a polyvalent cation (such as iron) complexed with a chelating agent (such as ethylenediaminetetraacetic acid or sodium salt thereof). In such a process, iron in the ferric state oxidizes the hydrogen sulfide to sulfur, the iron is reduced to the ferrous state, and the solution is regenerated by aeration to convert the iron to the ferric state. The sulfur is recovered from the solution by froth flotation, e.g., see Snavely et al, European Patent Application 82306861.4, filed on Dec. 22, 1982, Publication No. 0 086 919, published on Aug. 31, 1983.
U.S. Pat. No. 4,036,942 to Sibeud et al discloses a process for removing hydrogen sulfide and alkyl mercaptans from fluid streams by reacting the fluid streams with oxygen in the presence of a metal amino acid chelate in an aqueous solution containing an amine, resulting in the conversion of hydrogen sulfide to sulfur and alkyl mercaptans to dialkyldisulfides, and separating these from the aqueous metal chelate solution. However, the presence of oxygen in the reactants is disadvantageous in that it results in the conversion of some of the sulfur to sulfates and thiosulfates. Furthermore, the reaction requires a relatively long contact time between the metal chelate solution and the hydrogen sulfide-containing gas stream, such that if carbon dioxide is also present in the gas stream, the required contact time results in the absorption of carbon dioxide into the reaction solution, thus lowering the pH and lowering the efficiency of the reaction.
Diaz, U.S. Pat. No. 4,388,293, discloses a process of removing H.sub.2 S from a sour gaseous stream comprising contacting the gaseous stream with an aqueous reactant mixture comprising an oxidizing polyvalent metal chelate compound or a mixture of oxidizing polyvalent metal chelate compounds and a stabilizing amount of thiodiglycolic or 3,3-thiodipropionic acid. Elemental sulfur is removed from the reactant mixture, which is then regenerated with oxygen.
U.S. Pat. No. 4,009,251 to Meuly also discloses a process for removing hydrogen sulfide and alkyl mercaptans from gaseous streams by oxidizing the hydrogen sulfide to sulfur substantially without the formation of sulfur oxides, in the presence of a metal chelate catalyst solution and a soluble alkali metal, or alkaline earth metal or ammonium or amine salt of an acid having a pK within the range of about 1.2 to about 6. The spent metal chelate catalyst solution is then regenerated with oxygen. The alkyl mercaptans are oxidized to dialkyldisulfides under the same conditions. Meuly attempts to eliminate the oxidation of hydrogen sulfide to sulfur oxides by the addition of the above-described acid salts. Such addition to the metal chelate catalyst solution is required since Meuly reacts the hydrogen sulfide-containing gas stream with oxygen and recognizes that sulfur oxides may be formed by such a reaction mixture. Furthermore, the process of Meuly requires relatively long contact times for oxidation, and thus when carbon dioxide is present in the hydrogen sulfide-containing gas stream, the relatively long contact time also results in the absorption of CO.sub.2 and the consequential reduction in the pH of the solution and reduction in the efficiency of the system.
The entire contents of all of the above patents and publications are incorporated herein by reference.
In the processes utilizing polyvalent metal chelates, e.g., iron chelates, to remove H.sub.2 S from the gaseous stream, such as in the process of Snavely et al, discussed above, there is a continuous but steady loss of the chelated polyvalent metal from the process solution, thereby requiring the addition of the chelated polyvalent metal to the process solution to maintain the H.sub.2 S-oxidizing capacity thereof. Some of the workers in the art have contributed the loss of the chelated polyvalent metal to biodegradation, i.e., metabolism of the chelated polyvalent metal by bacterial agents. Based on this hypothesis, it was proposed to introduce a broad spectrum biocide into the process solution in an amount sufficient to act as a bacteriostat to significantly lower the rate of growth of the bacteria and other microorganisms in the reaction solution, e.g., see Primack et al, U.S. Pat. No. 4,455,287. Other workers in the art attributed the loss of chelated metal to the oxidation of the chelant.
However, a need still exists in the art to provide a more efficient, simple and economic process of preventing the loss of the chelated polyvalent metal from the reaction solution comprising a chelated polyvalent metal used to remove hydrogen sulfide from a gas.